Sunday, October 13, 2013

Geologic Wonders of the Bayou Country

I had a job once that had me on the road every day, driving through some of the most amazing landscapes in the country: the Colorado rockies, and the mountains and deserts of northern New Mexico. Behind the seat of my truck were two books, called Roadside Geology of Colorado, and Roadside Geology of New Mexico. Whenever I saw particularly stunning geologic formations--hanging glacial lakes, volcanic towers jutting from the desert floor, warped layers of rock in a cliff--I would check those books to read about how they formed. There's a whole series of Roadside Geology books, mostly covering the geologic wonderland that is the western United States.

So I was surprised when I moved to Louisiana and discovered there is a book called Roadside Geology of Louisiana. Louisiana is famous for many things, but stunning geologic formations are not one of them. Roadside geology? What roadside geology? But I decided to give the book a chance, and I'm glad I did. It contains a couple of the most amazing (and disconcerting) facts I've ever read. I learned, for example, how the land I live on here is younger than the Great Pyramid of Egypt, and how the Mississippi River has been trying to jump its banks and abandon New Orleans and Baton Rouge for decades.In the last 7500 years--the proverbial geologic blink of an eye--the river and its sediments have created all of southeast Louisiana, from New Orleans south. If you could watch a timelapse film of that period, you would see the river writhing back and forth like a loose fire hose, as it jumps from one channel to another every few hundred years. 

What happens is that the river builds great delta lobes of sediment out into the Gulf of Mexico, eventually lifting itself up until it flows above the surrounding landscape, contained within shallow natural levees when it isn't flooding. Eventually, it jumps those banks entirely and finds a lower path to the sea. The old channel becomes a slow moving bayou, and the old delta starts to erode. This has happened at least six times in the last 7,500 years, building southeast Louisiana in a series of overlapping sedimentary lobes. In the 1950's, the Army Corps of Engineers realized that the mighty Mississippi was getting tired of its old path, and would abandon it for the Atchafalaya Basin within decades. They've delayed this with some heroic engineering projects, but nobody knows how long those will hold out. As Mark Twain, no stranger to the great river, once wrote, "The Mississippi River will always have its own way. No engineering skill can persuade it to do otherwise."

The river's grand meanders are fascinating, but I'll write more about them another day. Today I want to focus on another of Louisiana's geologic wonders. If you make a pilgrimage to Avery Island, home of that most celebrated of condiments, Tabasco Sauce, you'll find that it's not actually an island. Instead, it's a short, round hill just over 150 feet tall and a couple of miles across. Not exactly a towering peak, but it does look odd emerging from the surrounding plains, which are billiard table flat. What's amazing about Avery Island is not its ruggedness, but the fact that it's made out of salt. That little hill is the top of salt dome; a 40,000 foot tower of almost-pure salt, poking straight through the surrounding sediments like a finger through a layer cake. Avery Island is actually one of the "Five Islands" of south central Louisiana, a chain of salt domes rising above the plains.

From Louisiana Department of Natural Resources
All these domes, as well as many others that don't quite reach the surface, have a history going back over 200 million years. During the Triassic Period, the supercontinent of Pangea began to break up. North and South America pulled apart, forming a shallow sea--the ancestral Gulf of Mexico. As sea levels rose and fell over millions of years, that sea sometimes dried up, leaving layers of evaporated salt behind. This formed a geologic stratum known as the Louann Salt, which in Louisiana lies under as much as 40,000 feet of newer sediment. 

But it doesn't always stay down there. The weird thing about salt is that when you put it under enormous pressure, it starts to act like a liquid. If there is a weak spot in the layers above it, the lighter salt will be squeezed upward like caulk, forming an underground tower of salt thousands of feet tall. 

That's a lot of salt, and salt is a valuable commodity. When I went to Avery Island last week, I saw there was a Cargill salt mine there, and a Morton salt mine on Weeks Island, a few miles away. Salt is impermeable--liquids and gases can't flow through it, as they can through porous rocks like sandstone. This means salt domes tend to trap oil and gas along their tops and sides, which is why you often find oil and gas wells dotting the landscape around them. The impermeability of salt domes is useful in other ways. Various industries have hollowed out great cavities in the salt to store oil, sludge, brine, and so on. The salt keeps these stores from flowing out into the surrounding rocks and water table. 

Well, that's how it usually works. There have been accidents. In August, 2012, one of these cavities collapsed in Assumption Parish. It was built too close to the edge of the salt dome, and the wall caved in. This caused a massive, flooded sinkhole to appear on the surface, along Bayou Corne. The sinkhole has grown to over 25 acres, occasionally swallowing whole stands of trees--slurping them right down into the ground. 

An even more spectacular accident happened in 1980 at Jefferson Island, a salt dome just north of Avery Island. A crew was working on an oil well on nearby Lake Peignour when their drill froze up. They heard a series of loud pops, and their oil rig started leaning to one side. Being men of good sense, they fled for the shore. Meanwhile, workers in the salt mine far below also heard loud noises, and saw water seeping into their mine. Also men of good sense, they got out of that mine as fast as they could--taking turns boarding an eight-man elevator while the mine filled with water. 

What seems to have happened is that the oil well drilled right into the salt mine, and pulled the plug on the lake. Before long a great maelstrom formed, and the workers on the shore watched as it swallowed the oil well, 11 barges, and a tugboat. A fisherman out on the lake barely escaped being sucked in, gunning his boat's engine against the current until he finally reached the shore. After a few hours, when all the fresh water was sucked from the lake, a canal connecting it to the Gulf of Mexico started flowing backward; dumping saltwater into the crater and forming a 150 foot waterfall that briefly held the record as the largest in Louisiana. After a while, 9 of the 11 barges popped back up to the surface. The oil rig was never seen again, and the Lake Peignour is now a saltwater lake. 

It turns out Louisiana geology isn't as boring as you might think. I highly recommend reading Roadside Geology of Louisiana, especially if you live here. If anyone sees it and scoffs, just tell them the story of Lake Peignour.

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